(1) Field of the Invention
The present invention relates to a solid-state imaging element having a photoelectric conversion element which is formed on a semiconductor substrate and an optical element which is formed on the photoelectric conversion element, and particularly to a solid-state imaging element that uses a photonic crystal as the optical element, and to a solid-state imaging device including a plurality of the solid-state imaging elements.
(2) Description of the Related Art
Conventionally, a variety of techniques for enhancing light condensing efficiency of solid-state imaging elements have been suggested. For example, the solid-state imaging element 30 shown in FIG. 1 not only has a microlens 21 formed on the top of the imaging element 30, but also an inner-layer lens 24 formed inside the imaging element 30 for condensing incident light into a photoelectric conversion element 27, in order to improve light condensing. In addition, the solid-state imaging element 30 is equipped with a color filter 22 for detecting a red, green or blue (RGB) light color signal. In other words, one of three types of color filters is selected for the solid-state imaging element 30 depending upon a color signal to be detected.
In addition, other various techniques concerning solid-state imaging elements (or solid-state imaging devices) have been suggested (See, for example, Japanese Laid-Open Patent Application Publication No. 2003-133536 (“the '536 Publication”) and Japanese Laid-Open Patent Application Publication No. 2001-44401 (“the '401 Publication)).
The solid-state imaging device disclosed in the '536 Publication has a structure for shading light of a predetermined wavelength range from a region surrounding a window on a photoelectric converter by providing in that region a photonic crystal having a waveguide structure. Therefore, this solid-state imaging device allows efficient light condensing while preventing the light from being incident to the peripheral region of the photoelectric converter, without using a shading layer made of metal or the like.
Furthermore, in the solid-state image pickup element disclosed in the '401 Publication, a tapered reflection film having a light reflecting characteristic is formed on a photoelectric conversion part. Transparent films, whose refractive indices are higher in the center than in the vicinity of the photoelectric conversion part, are formed on the photoelectric conversion part which is surrounded by the reflection film. As a result, the light incident to the solid-state image pickup element is condensed efficiently to the photoelectric conversion part and, thus, the light condensing is improved.
However, the above-mentioned solid-state imaging elements (or solid-state imaging devices) use a plurality of elements for light condensing and color separation, so it is impossible to enhance light condensing beyond certain limits due to reflection loss and coupling loss between respective elements. This problem presents an obstacle when miniaturizing a solid-state imaging element for increasing the number of pixels in every unit area.
There is another problem with the above-mentioned solid-state imaging elements. Since a color filter has to be provided separately, and the resins and pigments that make up the color filter are mere fractions of the size of the solid-state imaging element, color separation becomes more difficult as the solid-state imaging element becomes smaller.
There is still another problem. In a solid-state imaging device that includes a set of the solid-state imaging elements 30, light is incident almost perpendicularly upon the solid-state imaging elements 30 located around the center of the device, whereas it is incident obliquely upon the solid-state imaging elements 30 located on the periphery of the device. However, it is difficult to form the microlens 21 for the solid-state imaging elements 30 around the center of the device separately from the microlens 21 for the elements 30 on the periphery thereof (namely, to control the forming of each microlens 21 in consideration of the position of each solid-state imaging element 30 in the device) because the microlens 21 is formed by reflow process; thus, the light condensing on the periphery of the solid-state imaging device is lower than that in the center thereof.
Furthermore, since the solid-state imaging device as disclosed in the '536 Publication employs a method of obtaining each of RGB color signals of the light using a mirror, it is difficult to enhance light condensing beyond certain limits.